Coral larval proteomics at onset of metamorphosis highlights innate immunity maturation in parallel to neuro-sensing and skeletal development.

Introduction: Coral larval metamorphosis is a critical life cycle transition from swimming planula to benthic polyp, key for reproductive success and survival of reef populations. The larval physiological processes involved during this transition are relatively unknown, in the absence of exogenous microbial induction. Methods: Here the identity and abundance of coral proteins detected at onset of metamorphosis (swimming planula to 'settler' stages) was investigated using planulae released from 4 distinct brooding Pocillopora acuta coral colonies, to consider maternal carry-over effects on the variability of larval proteomes. Results and discussion: NanoLC-MS/MS data analysis identified a total of 5,570 coral proteins, of which 1,119 occurred only either in planula or settler. Label-free quantification revealed 102 differentially enriched proteins (DEPs, log2 fold change > |2| p-value < 0.05), categorized into 7 predicted functional groups: adhesion and cytoskeleton remodeling, neurosensing, biocalcification toolkit, metabolism, morphogenesis, innate immunity, and antioxidant stress defense. Additionally, 98 aboral unique proteins and 25 aboral DEPs were quantified in the bisected planula, consistent with the presence of specialized aboral cell types involved in sensing of environmental cues. These results reveal the activation of coral innate immunity during larval metamorphosis, providing better knowledge of coral settlement physiology, with potential future ecological applications.